The present invention relates to tool holders for chip removing machining provided with an outermost thin film interference color layer.
The present invention relates to coated cutting tool inserts provided with an outermost thin film interference color layer. As used herein, the term “insert” refers also to solid carbide drills, end mills and the like.
Cemented carbide cutting tool inserts coated with various types of hard coatings have been commercially available for years. Such coatings are generally built up by several hard layers like TiC, Ti(C,N), TiN and Al2O3 in a multilayer structure. The sequence and the thickness of the individual layers are carefully chosen to suit different cutting applications and work-piece materials. Such coatings are most frequently deposited by using Chemical Vapor Deposition (CVD), Moderate Temperature Chemical Vapor Deposition (MTCVD) or Physical Vapor Deposition (PVD) techniques. In some rare cases also Plasma Assisted Chemical Vapor Deposition (PACVD) has been used.
The CVD technique employed for coating inserts is conducted at a rather high temperature, about 900-1000° C. Due to this high deposition temperature and to a mismatch in thermal expansion coefficient between the deposited coating materials and the cemented carbide insert, CVD produces coatings with cooling cracks and tensile stresses. With the CVD-technique it is possible to deposit many hard and wear resistant coating materials like Al2O3, TiC, Ti(C,N), TiN, Ti(C,N,O) and ZrO2. The microstructure and thereby the properties of these coatings can be altered quite considerably by varying the deposition conditions.
The PVD technique runs at a significantly lower temperature about 450-700° C. and it is performed under ion bombardment leading to high compressive stresses in the coating and no cooling cracks. Because of these process differences, CVD-coated inserts are more brittle and thereby possess a lower toughness behavior compared to PVD coated inserts. PVD-coatings are generally used when sharp edges are desired.
Although these coatings have excellent technological properties, they often unfortunately have a dull and somewhat unattractive appearance. For that reason, often a thin decorative layer of TiN is deposited as the outermost layer giving the insert an attractive golden color. It would, of course, be very interesting if inserts could be given different colors as a guide to the end user to select the correct grade for a certain machining operation and not only for decorative reasons. Unfortunately, the number of colored compounds suitable for such purpose is limited. It is also important that said coloring layer does not negatively influence the properties of the inserts. The coloring layer should also be possible to deposit without introducing further unwanted compounds into the reactor. For drills and end mills, its is also important that the they easily can be recoated after regrinding with a new functional wear resistant layer without prior decoating. It is also important that the colors obtained are consistent both within a charge and between charges.
GB 1389140 discloses color coding of sintered hard metal cutting inserts by coating the inserts with a layer of TiC and at least one outer layer of TiN and/or TiCN which is no thicker than 3 μm. The coatings are formed by chemical vapor deposition.
U.S. Pat. No. 5,700,569 discloses an alumina coated cemented carbide insert having improved properties for metal cutting applications. Varying the number of alumina layers changes the color of the coated insert from green at 15 layers to blue at 32 layers.
JP-A-2001-341005 discloses a insert coated with a TiCN-layer with a color defined in terms of its chromaticity a* being greater than 0 but less than 10 (red direction), b* being greater than 0 but less than 20 (yellow direction) and a brightness of L* being greater than 0 but less than 100. The a*, b* and L* coordinates are well-known in the art as part of the CIELab system, a uniform device independent color space in which colors are located within a three-dimensional rectangular coordinate system. The three dimensions are lightness (L), redness/greenness (a) and yellowness/blueness (b).